gscreen config -add 'tester' config to aid in testing custom screens

if a user copies his glade file as tester.glade into this folder,
then loads tester in linuxcnc the gladefile will display as a linuxcnc
screen. If the gladefile use emc_actions then it can interact with linuxcnc.
gladevcp -make action_open sensitive to a running machine

shouldn't load a new Gcode file while linuxcnc is running a current one.
So we desensitize the button